"Katrina was on her way to becoming one of the leading barristers at the bar.

"She was also a dear friend to all of us and will be deeply missed. Our heartfelt condolences to all her family and friends.

"We also send our condolences to everyone affected by this tragic event. We thank all those who have sent messages of support."

Belinda Berryman, a school friend of Ms Dawson, told 702 ABC Sydney she is thinking of Ms Dawson's family and wanted to honour her memory.

"She really stood out as a very warm, happy, friendly person, she was someone you thought would always do well in the world," she said.

"It's shocking anyway but it's just so hard to comprehend that it was someone you knew - it really brings it home.

"We always say people are amazing but Katrina, she was a really, really sweet girl.

"I just wanted to honour her memory to say that we're thinking of them (Ms Dawson's family) and I'll always think of her and her family on this day every year."

Ascham School, which Ms Dawson attended and where one of her daughters is a pupil, issued a statement describing Ms Dawson as a well-respected and giving member of the school community.

The statement said Ms Dawson attended Ascham between 1981 and 1994 and had a stellar school career.

"Katrina's daughter Chloe is an Ascham student and her younger daughter Sasha starts with us next year," the statement said.

Ascham's Head of School Andrew Powell said Ms Dawson was dux of Ascham School and a prefect.

"I knew Katrina well," he said.

Our hearts go out to Katrina's entire family... We have been greatly shocked and saddened by this tragic news and will do everything possible to support the family at this terrible time.

Ascham's Head of School, Andrew Powell

"Katrina is remembered affectionately as an outstanding student who excelled in all aspects of school life and was much-loved by her peers and teachers alike.

"She was generous in giving of her time and this year Katrina used her skills as a barrister to guide our senior students in preparing for mock trials.

"Our hearts go out to Katrina's entire family, including her husband Paul, and her children, Chloe, Sasha, and Oliver. We have been greatly shocked and saddened by this tragic news and will do everything possible to support the family at this terrible time."

Dozens of Muslim groups have issued a joint statement expressing sympathy for the families of those killed and injured during the siege.

"We pray for the speedy recovery of all those injured and traumatised," Australia's Grand Mufti, Ibrahim Abu Mohammad, said in the statement.

"We are confident that with a continued display of responsibility and restraint, community harmony will be maintained."

Lindt and Sprüngli, which owns the cafe, released a statement on Facebook thanking NSW Police and offering support to the siege victims and their families.